linux-ext4 - Re: [PATCH] fscrypt: add a documentation file for filesystem-level encryption

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Message-ID: <20170831175046.GB5023@gmail.com>
Date:   Thu, 31 Aug 2017 10:50:46 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Anand Jain <anand.jain@...cle.com>
Cc:     linux-fscrypt@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Richard Weinberger <richard@....at>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] fscrypt: add a documentation file for filesystem-level
 encryption

Hi Anand,

On Mon, Aug 28, 2017 at 08:18:46PM +0800, Anand Jain wrote:
> 
> 
> On 08/23/2017 01:07 AM, Eric Biggers wrote:
> >On Tue, Aug 22, 2017 at 11:33:51PM +0800, Anand Jain wrote:
> >>
> >>
> >>On 08/22/2017 10:55 AM, Eric Biggers wrote:
> >>>On Tue, Aug 22, 2017 at 10:22:30AM +0800, Anand Jain wrote:
> >>>>
> >>>>Hi Eric,
> >>>>
> >>>>   How about a section on the threat model specific to the file-name ?
> >>>>
> >>>>   (Sorry if I am missing something).
> >>>>
> >>>>Thanks, Anand
> >>>
> >>>It's already mentioned that filenames are encrypted: "fscrypt protects the
> >>>confidentiality of file contents and filenames in the event of a single
> >>>point-in-time permanent offline compromise of the block device content."
> >>>There's not much more to it than that; all the other points in the "Threat
> >>>model" section (offline manipulations, timing attacks, access control, key
> >>>eviction, etc.) are essentially the same between contents and filenames
> >>>encryption.
> >>
> >>  Do you think if application does not keep the sensitive information
> >>in the file-name, would that remove the file-name from the list of
> >>items that should be protected ?
> >>
> >
> >If *no* applications care whether the filenames are encrypted or not, sure.
> >But are you absolutely sure that no applications care?  How do you know?  And what
> >is the advantage of not encrypting the filenames anyway?  It is better to
> >encrypt by default.
> >
> >Eric
> 
>  (sorry for the delay in reply due to my vacation).
> 
>  It all depends on the use case, Android is one such use case. Some
> data center use a known set of application. Again it all depends on
> the use case.
> 
>  File-name is a kind of File-system semantic and altering based on
> the on the user key context does not guarantee the system will be
> compatible with all their legacy applications.
> 

You really need to give more detail about why filename encryption specifically
is a problem.  What "legacy applications" is a problem for, and why?  What do
you mean by a "data center" use case?  Isn't contents encryption a "problem" for
some "legacy applications" as well?  Contents encryption changes filesystem
semantics as well.

>  Also a section on backup and restore in this doc will be a good
> idea. As I think that will be affected IMO. And needing to have the
> user master key to restore encrypted file isn't practical in some
> data center multi tenanted solutions. Albeit it may work in some
> cases but hard to generalize.
> 

The proposed documentation already mentions that there is no way to backup the
raw ciphertext currently.  I'll try to make this a bit clearer, but in any case
I am documenting the current state of things; the new documentation file is
*not* a plan for things that don't exist yet.  If/when someone proposes a
patchset which adds a backup+restore API they can propose a documentation update
along with it.

Eric